The use of the drug Prempro, a combination of two female hormones that many women take after menopause, is called hormone replacement therapy (HRT). In July 2002, a medical article reported the results of a study that was done to determine the effects of Prempro on many diseases. Writing Group for the Women’s Health Initiative Investigators, “Risks and Benefits of Estrogen Plus Progestin in Healthy Postmenopausal Women,” JAMA 388 [2002]: 321–33) The study was placebo-controlled, randomized, and doubleblind. From studies like these, it is possible to make statements about cause and effect. The figure shows comparisons of disease rates in  the study. 

a. For which diseases was the disease rate higher for those who took HRT? And for which diseases was the rate lower for those who took HRT?

b. Why do you suppose we compare the rate per 10,000 women (per year), rather than just reporting the numbers of women observed who got the disease?

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40 35 Effects of HRT 30 25 20 15 HRT Placebo 10 5 Heart Disease Stroke Pulmonary Embolism Breast Cancer Endometrial Cancer Colorectal Cancer Hip Fracture Rate per 10,000 Women per Year